In cold weather, say minus 10.degree. C. and below, most motor vehicles fail to start when their starter is actuated. The vehicle-equipped battery fails because the lubricating oil is too viscous and because of the reduced power output of the battery. It is a conventional procedure to connect in parallel an additional battery to the vehicle battery to thus start the engine. Such a solution requires an extra battery, which has to be kept in a warm environment. It requires also opening the vehicle hood and to manually connect the two batteries with the risk of damaging the batteries when improperly connected.